Satellite digital audio radio services (SDARS) broadcast audio programming directly from a satellite to an end user's radio receiver so that a typical SDAR broadcast reaches an extensive, diverse, geographical region. In order to ensure high quality, uninterrupted transmission in all the reception regions reached by the broadcast, SDAR providers typically complement their satellite broadcast with gap-filling rebroadcasts using terrestrial stations located in regions having poor or no satellite reception, such as cities with tall buildings. The signals broadcast from the satellite and by the terrestrial stations contain the same audio data, and are typically on adjacent frequencies but use different coding techniques. The terrestrial signals are also typically broadcast at significantly higher signal strength, primarily because terrestrial stations have easy access to electrical power while satellites are limited to the electrical power available from their solar panels.
FIG. 1 is a schematic drawing showing an exemplary SDAR system 10 provided by Sirius Radio Systems of New York, N.Y., which broadcasts over one-hundred channels of audio programming directly from satellites to users equipped with appropriate receivers. Two geo-synchronous satellites 12 and 14 transmit time division multiplexed (TDM) signals 16 and 18 directly to the end user's receiver 20 using two S band (2.3 GHz) frequencies. The end user's receiver 20 is typically a mobile receiver in an automobile or a truck. In regions with poor satellite reception, terrestrial repeater stations 22 broadcast a coded orthogonal frequency division multiplexed (COFDM) signal 24 containing the same audio data as that broadcast in the satellite signals. The terrestrial COFDM signals 24 are broadcast at an S band frequency, lying between the frequencies of the two, satellite TDM signals 16 and 18, and at a significantly higher power level.
FIG. 2 shows a schematic diagram of a prior art, digital radio receiver designed to receive and decode the audio channels contained in the Sirius system signals. The receiver 26 has two decoding circuits 28 and 30, one for receiving TDM signals directly from the satellites and one for receiving COFDM signals. The TDM decoding circuit 28 has a TDM antenna 32 for receiving the signal, which is then amplified by TDM RF amplifier 34 and the TDM IF amplifier 36. The amplified signal is digitized by a TDM analogue-to-digital converter (ADC) 38. The digitized TDM signals are down-converted by TDM digital-down-converter (DDC) 40, before being demodulated. In the Sirius system, one geo-synchronous satellite has a version of the signal that is delayed by four seconds, so there are two TDM demodulators 42 and 44, one for handling the un-delayed signal and one for handling the delayed signal.
The ADC 38, which is typically a 10 bit device with a usable dynamic range of about 60 dB, plays an important role in digital radio reception. As long as the digitized signal is an accurate representation of the incoming analogue signal, digital filtering techniques make it possible to extract very weak signals, such as those received from a satellite, even in the presence of a significant amount of noise. Accurate digitization requires that the incoming signal is amplified sufficiently to fill as much of the ADC's dynamic range as possible. It is, however, also very important not to over amplify the incoming signal since, when the ADC is overdriven and overflows, a small signal in a noisy background can be completely lost. This happens because the ADC simply truncates any excess signal.
The appropriate gain setting of amplifiers 34 and 36 that amplifies the incoming signal to the optimal level for the ADC is controlled by the TDM automatic gain control (TDM AGC) 48. The TDM AGC monitors the demodulated TDM signals TDM1 and TDM2, and uses the stronger of the two demodulated TDM signals, selected by the Max selector 46, to set the gain of amplifiers 34 and 36 so that the portion of the received signal containing the best TDM signal is amplified appropriately, and a constant volume output is obtained.
Any available COFDM signal is demodulated using a parallel COFDM decoding circuit 30, having COFDM antenna 50, COFMD RF amplifier 52, COFMD IF amplifier 54, COFDM ADC 56, COFDM digital down converter 58, COFDM demodulator 60, and COFDM AGC 60.
In prior art receivers designed for the Sirius system, the front end of both the TDM and the COFDM decoding circuits 28 and 30 contain substantially identical components, i.e., the TDM and COFMD antennas 32 and 50, the TDM and COFDM RF amplifiers 34 and 52, the TDM and COFMD IF amplifiers 36 and 54 and TDM and COFMD ADCs 38 and 56 are the same as each other. In order to reduce the power requirements and the cost of receivers, it is highly desirable to have a receiver with only one front-end, i.e., only one antenna, one RF amplifier, one IF amplifier and one analogue-to-digital converter (ADC).
Practical implementation of a single front-end circuit is not, however, simple. A major problem in such a circuit is that the amplifier gain settings for the two types of signal may be incompatible with each other. This causes difficulties if the amplifier gains are controlled using a simple, two-state AGC, with one state to optimize the gain for a COFDM signal and one state to optimize the amplifier gain for a TDM signal. In such a system, the overall gain of the front-end amplifiers that is optimal for the weak TDM signals from the satellites typically will over-amplify the incoming COFDM signal from the terrestrial stations, resulting in the COFDM signal over-flowing the ADC's dynamic range. This over-flow of the ADC's dynamic range means that the demodulated COFDM audio data is of very poor quality, and may even be non-existent. The receiver may be “blinded” to the presence of a good COFDM signal and simply stick with a poor quality TDM signal until the TDM signal is completely lost.
Similarly, if the amplifiers gain settings are optimal for the ADC to digitize the portion of the signal containing the stronger, COFDM signal, the portion of the signal containing the TDM signal will be under-amplified, and poorly digitized by the ADC. The result is that if the receiver does lock on to a terrestrial COFDM signal, it may stay locked onto the terrestrial signal, even if there is a better satellite signal available.
In order to achieve the highly desirable power and cost savings that a single power amplifier and ADC would provide, it is necessary to have an automatic gain control that can adjust the amplifiers gains in a way that makes it possible to use the best available signal, and not to be blinded to the availability of a better signal by either under or over amplifying any portion of the signal with respect to the ADC dynamic range.